1. Field of the Invention
This invention relates to a process for producing a resin bonded magnet structure, such as one used for a rotor in motors, by the direct formation of a permanent magnet on the outer surface of a supporting member. More particularly, it relates to a process for producing a resin bonded magnet structure by fixing a melt spun powder of a rare earth element-iron alloy with a resin so as to have a bulk-like shape, and then directly integrating the fixed melt spun powder with a supporting member.
2. Description of the Prior Art
A metastable permanent magnetic material can be obtained by ultra-quenching a rare earth element-iron alloy with a melt spinning technique. The resulting permanent magnetic material has a R.sub.2 TM.sub.14 B phase with a grain size of 20 to 400 nm (wherein R is Nd and/or Pr, and TM is Fe and/or Co), and an amorphous phase (R. W. Lee, Appl. Phys. Lett. Vol. 46(8), 15 April (1985), p. 790).
The material is magnetically isotropic and has a relatively high residual induction of, typically, 7.5 kG or more. However, because the permanent magnetic material obtained by a melt spinning technique is a powder in the form of thin ribbon or flake, it must be fixed by a certain method to form a bulk-like permanent resin bonded magnet such as one used in a motor. In general, it is known that the melt spun powder is fixed with a resin to form a bulk-like resin bonded magnet.
The first example of the bulk-like resin bonded magnet applied to motors is a ring-shaped resin bonded magnet with a small diameter, which is used as a rotor in permanent magnet (PM) type step motors for office automation systems. The reason for this application is as follows.
In order to obtain a ring-shaped resin bonded magnet, such as one made of an Sm-Co alloy, having a radial magnetic anisotropy and a high degree of orientation, the ring-shaped resin bonded magnet should be prepared by charging a compound made of Sm-Co alloy powder and resin into a cavity, and then introducing into the cavity a given amount of magnetic fluxes produced by an exciting coil around a yoke. However, in the case of a ring-shaped resin bonded magnet with a smaller diameter, when the magnetic fluxes are introduced into the cavity, a significant amount of magnetomotive forces will be consumed as leakage fluxes because of a smaller diameter of the cavity. Thus, according to this procedure, it is not possible to obtain a higher orientation and therefore the resulting ring-shaped magnet with a smaller diameter cannot be used for producing a high-torque compact motor.
The second example of the bulk-like resin bonded magnet applied to motors is a ring-shaped resin bonded magnet with a relatively large diameter, such as one used in a brushless motor for home appliances with an output power of several watts, in which a sintered ferrite magnet has been widely used so far. In this case, the ring-shaped resin bonded magnet is used as a rotor which passes through an axis and is held by a magnetic supporting member such as laminated cores. The reason for this application is as follows.
For example, the ring-shaped resin bonded magnet structure with a relatively large diameter is produced by the steps of: (1) adding a solid epoxy resin and a microcapsule which contains at least one liquid epoxy resin to a melt spun powder of a rare earth element-iron alloy to form a granulated intermediate material; (2) mixing the granulated intermediate material with a curing agent and a lubricant to form a compound; (3) filling the compound into a cavity in which a magnetic supporting member has been provided; and (4) compressing the compound together with the magnetic supporting member. The resulting ring-shaped resin bonded magnet structure can be used directly as a rotor in brushless motors. Thus, the ring-shaped resin bonded magnet structure with a relatively large diameter has complied with the requirement of a high-torque compact motor, while keeping at least the total cost of motor production from increasing.
In addition to the excellent magnetic properties, a ring-shaped resin bonded magnet with a relatively large diameter has the following advantages: (1) a ring-shaped resin bonded magnet with, for example, an outer diameter of about 50 mm and a thickness of about 1 mm can readily be produced by compressing a compound together with a supporting member at room temperature; (2) because a ring-shaped resin bonded magnet is attached directly to the magnetic supporting member without forming any bonding layer, its permeance coefficient can be improved; (3) a process for bonding a ring-shaped resin bonded magnet with a supporting member can be eliminated; and (4) a process for correcting the rotation balance of a rotor can also be eliminated.
However, in order to further apply a ring-shaped resin bonded magnet to a rotor in various motors for home appliances with a higher output power, e.g., brushless motors for home appliances and servomotors for factory automation systems with an output power of from tens of watts to hundreds of watts, it has been required to solve the following problems: an improvement in the residual induction of resin magnets by densification, and a stabilization of its higher level; an improvement in the integration strength of resin magnets with supporting members, and a stabilization of its higher level; an improvement in the shape flexibility for the formation of resin magnets having different shapes such as a thick ring or a deformed ring; and an achievement of the dimensional precision of resin magnets.